(2) The law of addition and association: When three numbers are added, the first two numbers are added first, or the last two numbers are added first, and the sum remains unchanged. (a+b)+c=a+(b+c)
3 The essence of subtraction: A: Consecutive subtraction of two numbers equals subtraction of the sum of these two numbers. a-b-c=a-(b+c)
B: Subtract the difference between two numbers. It is equal to subtracting the minuend in the difference and adding the minuend in the difference, a-(b-c) = a-b+c.
(4) Multiplication and exchange law: When two numbers are multiplied, the position of the exchange factor remains unchanged. a*b=b*a
⑤ Multiplication and association law: When three numbers are multiplied, you can multiply the first two numbers or the last two numbers first, and the product is unchanged. (a*b)*c=a*(b*c)
⑥ Multiplication and distribution law: When the sum of two numbers is multiplied by a number, you can multiply it with this number first, and then add the products. a*(b+c)=a*b+a*c
⑦ The essence of division:
A quotient invariance: the dividend and divisor expand or shrink by the same multiple at the same time, except for 0, the quotient remains unchanged.
B divided by two numbers is equal to the product of these two numbers. a÷b÷c=a÷(b×c)